Well I read the full paper and I must say, it's a difficult topic. Those who have personally asked me how you can actually measure something being in two states at the same time will hopefully find an answer here.
Quantum mechanics has been well established by experiment. The theory behind this systems involved in this experiment are well understood. That being said, it is a comparison between the gathered data and what they theory predicts that leads the physicists who performed the experiment to conclude that they are observing a macroscopic quantum effect (the drum both vibrating and not at the same time.). Let me try to explain.
The experimental set up is a extremely cold piezoelectric oscillator electrically coupled to a quantum bit (or, qubit). If none of those words make sense, let me break it down for you. A piezoelectric material is a material that produces electricity from being bent or vibrated. You have seen piezoelectric materials in many places, perhaps the most familiar being in a butane lighter. When you click the lighter on, you are actually compressing a piezoelectric element that makes a little spark. The next word I would like to explain is the oscillator. An oscillator is something that vibrates in a predictable manor. Think of the surface of a drum. When you strike a drum, the surface moves up and down at a predictable rate - a drum is an oscillator. In this experiment, the piezoelectric oscillator is just a little piece of metal that produces an electric current when it vibrates. The physicists know at what rate (frequency) it can vibrate.
Next up is the qubit. A quantum bit is essentially a 1 or a 0 that obeys the laws of quantum mechanics. That is, a qubit is in two states simultaneously until measured. I do not pretend to be an expert in quantum computers, for some more information, I recommend Michael Nielsen's quantum computing for everyone. The basic idea behind this experiment is that when the oscillator vibrates, it affects the qubit. This allows the two systems to be entangled. Knowing the state of one means you know the state of the other. This way, the oscillator can be isolated (so that nothing is allowed to interact with it), and it's state can be inferred by qubit measurements. Pretty cool.
Now, the most important part of the experiment is the cooling. The colder something is, the less it's molecules that make it up move, the less they move, the lower the energy. They cooled the oscillator down to EXTREMELY low temperatures so that it would have the lowest possible energy it can have (called it's ground state.). This corresponds it the lowest possible rate at which it can vibrate. They made measurements of the ground state vibration rate and the next to the lower vibration rate. The qubit's state was either 0 for the ground state or 1 for the first excited state.
Here's the part that's difficult to swallow. We can't actually look at the oscillator while we run this experiment because this would count as a measurement. Remember, measurements collapse the wave function to a single outcome. This is boring and there's no way we can observe quantum behaviour if we are constantly observing something. So how did they know they observed a large object obeying quantum mechanics? Well, they repeated certain measurements on the qubit. By repeating the measurements, they were able to measure the probability and the state transition times for the qubit. By comparing these probabilities and transition times to both the theoretical and experimental predictions of the qubit's behavior, they determined that they qubit was in a quantum superposition state (both 1 and 0 at the same time.). But, the qubit and the oscillator are linked, so knowing the state of one tells you the state of the other! Thus, they report with high confidence that the oscillator, which would be actually visible with the naked eye (but still very tiny) was in a quantum superposition state (being both in the ground state and first excited state at the same time!).
This is a major discovery! Provided others can replicate similar results in different experiments, this will confirm that everything, not just small objects obey the laws of quantum mechanics. Even trillions of atoms cannot escape the strangeness of the quantum world.
Comments? Questions? Mistake in my post? Open discussion encouraged!
Up next, the quantum mechanical spring, then a discussion and some demonstrations of expectation values!
You know, that really does blow my mind! Holy cow that is really strange. First question though...what exactly makes up a qubit? I mean, how would we go about acquiring one? And why doesn't measuring the qubit count as measuring the oscillator? They are entangled right? And just how do we go about getting something entangled with something else? This is just all such a mystery to me!
ReplyDeleteFor your first question, I would direct you here.
ReplyDeleteMeasuring the qubit DOES count as measuring the oscillator, that's actually the point. Repeated measurements on the qubit defines the probabilities for the qubit, and thus for the oscillator. It's not about seeing it doing both at once, it's about measuring an entangled state on the qubit by repeated measurements.
Dan, you explain the discrete parts here well. I think more needs to be done towards the end, which is where you give us the payoff--that this means that all materials follow QM.
ReplyDeleteI think this is a pattern in your writing--you tell us the significance of all of the details at the end. I'd like you to encourage to add a revision step to your posts, where you take the big context and move it to the top of the post, so that we understand why all the details are important. You may not be able to write this way first thing, but over time you may become more used to giving us the payoff up front so that the details take on more significance.